In oil wells and gas wells (hereinafter, collectively referred simply as “oil wells”) of crude oil, natural gas, and the like containing H2S, sulfide stress-corrosion cracking (hereinafter, referred to as “SSC”) of steel in wet hydrogen sulfide environments poses a problem, and therefore oil country tubular goods excellent in SSC resistance are needed. In recent years, the strengthening of low-alloy sour-resistant oil country tubular goods used in casing applications has been advanced.
The SSC resistance deteriorates sharply with the increase in steel strength. Therefore, conventionally, steel materials capable of assuring SSC resistance in the environment of NACE solution A (NACE TM0177-2005) containing 1-bar H2S, which is the general evaluation condition, have been steel materials of 110 ksi grade (yield strength: 758 to 862 MPa) or lower. In many cases, higher-strength steel materials of 125 ksi grade (yield strength: 862 to 965 MPa) and 140 ksi grade (yield strength: 965 to 1069 MPa) can only assure SSC resistance under a limited H2S partial pressure (for example, 0.1 bar or lower). It is thought that, in the future, the corrosion environment will become more and more hostile due to larger depth of oil well, so that oil country tubular goods having higher strength and higher corrosion resistance must be developed.
The SSC is a kind of hydrogen embrittlement in which hydrogen generated on the surface of steel material in a corrosion environment diffuses in the steel, and resultantly the steel material is ruptured by the synergetic effect with the stress applied to the steel material. In the steel material having high SSC susceptibility, cracks are generated easily by a low load stress as compared with the yield strength of steel material.
Many studies on the relationship between metal micro-structure and SSC resistance of low-alloy steel have been conducted so far. Generally, it is said that, in order to improve SSC resistance, it is most effective to turn the metal micro-structure into a tempered martensitic structure, and it is desirable to turn the metal micro-structure into a fine grain structure.
For example, Patent Document 1 proposes a method which refines the crystal grains by applying rapid heating means such as induction heating when the steel is heated. Also, Patent Document 2 proposes a method which refines the crystal grains by quenching the steel twice. Besides, for example, Patent Document 3 proposes a method which improve the steel performance by making the structure of steel material bainitic. All of the object steels in many conventional techniques described above each have a metal micro-structure consisting mainly of tempered martensite, ferrite, or bainite.
The tempered martensite or ferrite, which is the main structure of the above-described low-alloy steel, is of a body-centered cubic system (hereinafter, referred to as a “BCC”). The BCC structure inherently has high hydrogen embrittlement susceptibility. Therefore, for the steel whose main structure is tempered martensite or ferrite, it is very difficult to prevent SSC completely. In particular, as described above, SSC susceptibility becomes higher with the increase in strength. Therefore, it is said that to obtain a high-strength steel material excellent in SSC resistance is a problem most difficult to solve for the low-alloy steel.
In contrast, if a highly corrosion resistant alloy such as stainless steel or high-Ni alloy having an austenitic structure of a face-centered cubic system (hereinafter, referred to as an “FCC”), which inherently has low hydrogen embrittlement susceptibility, is used, SSC can be prevented. However, the austenitic steel generally has a low strength as is solid solution treated. Also, in order to obtain a stable austenitic structure, usually, a large amount of expensive component element such as Ni must be added, so that the production cost of steel material increases remarkably.
Manganese is known as an austenite stabilizing element. Therefore, the use of austenitic steel containing much Mn as a material for oil country tubular goods in place of expensive Ni has been considered. Patent Document 4 discloses a steel that contains C: 1.2% or less, Mn: 5 to 45%, and the like and is strengthened by cold working. Also, Patent Document 5 discloses a technique in which a steel containing C: 0.3 to 1.6%, Mn: 4 to 35%, Cr: 0.5 to 20%, V: 0.2 to 4%, Nb: 0.2 to 4%, and the like is used, and the steel is strengthened by precipitating carbides in the cooling process after solid solution treatment. Further, Patent Document 6 discloses a technique in which a steel containing C: 0.10 to 1.2%, Mn: 5.0 to 45.0%, V: 0.5 to 2.0%, and the like is subjected to aging treatment after solid solution treatment, and the steel is strengthened by precipitating V carbides.